OMAP2: avoid descending into disabled framebuffer dirs

Rather than always add the omap2 dirs to the build list (and thus
force everyone to generate a useless built-in.o), bind the dirs to
their relevant kconfig symbol.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>